As a rule, a processing apparatus for photographic sheet material comprises several vessels each of which contains a treatment liquid, such as a developer, a fixer and a rinse liquid. As used herein, the term "sheet material" includes not only photographic material in the form of cut sheets, but also in the form of a web unwound from a roll. The sheet material to be processed is transported through these vessels in turn, by transport means such as one or more pairs of drive rollers, and thereafter optionally to a drying unit. The time spent by the sheet material in each vessel is determined by the transport speed and the dimensions of the vessel in the sheet feed path direction.
In a conventional processing apparatus the sheet material is transported along a generally horizontal feed path, the sheet material passing from one vessel to another usually via a circuitous feed path passing under the surface of each treatment liquid and over dividing walls between the vessels.
In a system for the development of aluminium lithographic printing plates of the type disclosed in EP-A-410500 (Agfa Gevaert NV), the apparatus comprises a housing with pairs of processing rollers carried on roller shafts supported within the housing. The processing rollers are positioned substantially parallel and in line contact with each other. Means are provided for feeding photographic sheet material between the rollers. The roller shafts are biased towards each other to exert a pressure on the photographic sheet material as it passes between the rollers. Processing machines having a substantially vertical orientation have also been proposed, in which a plurality of vessels are mounted one above the other, each vessel having an opening at the top acting as a sheet material inlet and an opening at the bottom acting as a sheet material outlet or vice versa. In the present context, the term "substantially vertical" is intended to mean that the sheet material moves along a path from the inlet to the outlet which is either exactly vertical, or which has a vertical component greater than any horizontal component. The use of a vertical orientation for the apparatus leads to a number of advantages. In particular the apparatus occupies only a fraction of the floor space which is occupied by a conventional horizontal arrangement.
Furthermore, the sheet transport path in a vertically oriented apparatus may be substantially straight, in contrast to the circuitous feed path which is usual in a horizontally oriented apparatus. As a consequence of the straight path, the material sensitivity to scratches becomes independent of the stiffness and thickness of the material.
In a vertically oriented apparatus, it is important to avoid, or at least minimise leakage of treatment liquid from one vessel to another and carry-over as the sheet material passes through the apparatus. U.S. Pat. No. 4,166,689 (Schausberger et al. assigned to Agfa-Gevaert AG) describes such an apparatus in which liquid escapes form the lower opening and is intercepted by the tank of a sealing device with two squeegees located in the tank above a horizontal passage in line with the lower opening. One or more pairs of drive rollers in the vessel close the lower opening and also serve to transport the sheet material along a vertical path which extends between the openings of the vessel.
In both such forms of processing apparatus, the rollers are used in pairs, biased towards each other, between which the sheet material passes to act as a seal between treatment vessels of the processing apparatus, that is to remove excess treatment liquid from the sheet as it passes from one treatment vessel to the next. This reduces carry-over of treatment liquid and thereby reduces contamination and wastage. A good removal of processing liquid is also required to reduce the drying time of the sheet material after the last process bath, and hence to reduce the energy use.
It is often convenient that these rollers also act as drive rollers, serving to advance the sheet material through the apparatus. To meet these demands successfully, the resilience of the rollers is important. Usually such rollers comprise a rigid core having a layer of, for example, elastomeric material positioned over the core. If the elastomeric material is too hard, the squeegeeing properties beyond the edges of the sheet material may not be optimum, resulting in an unacceptable level of carry-over. On the other hand, if the elastomeric material is too soft it will often contain oily materials which are liable to leach out of the elastomer and contaminate the sheet material, while the elastomeric material becomes progressively degraded.
Typical rollers have a core provided with a covering of elastomeric material. As the sheet material leaves a given liquid treatment vessel it is necessary to remove any liquid carried on the sheet material as efficiently as possible, to prevent carry-over of liquid into a next treatment vessel and to reduce edge effects which arise from non-homogeneous chemistry on the sheet material after squeegeeing. This applies whether the apparatus is of a horizontal or vertical configuration. To do this job properly, the rollers must exert a sufficient and homogeneous pressure over the whole width of the sheet material. Also, to reduce edge effects, it is desirable that the opposite roller surfaces are in contact with each other beyond the edges of the sheet material. To put this problem in context, rollers used in conventional processing apparatus for example have a length of 400 mm or more and a diameter of from 24 to 60 mm. The sheet material typically has a width of from a few millimetres up to 2 m and a thickness of 0.05 mm to 0.5 mm.
In view of the nature of elastomeric material, it is in fact impossible to totally eliminate any gap between the roller surfaces at the edges of the sheet material as it passes through the nip. It is desirable that the roller surfaces be in contact with each other within as short a distance as possible from the edges of the sheet material i.e. that the size of the leak zone should be minimised. It is important however that the force between the rollers is sufficient to prevent leakage when no sheet material is passing through. However, the force must not be so high as to risk physical damage to the sheet material as it passes through the nip, such as dislocation of coatings carried on the sheet material or separation of layers in the case of laminated sheet materials. Physical damage is more likely to occur with thin (for example paper) based sheet materials rather than thick (for example aluminium or polyester) based she et materials.